Conventionally, coated steel sheets to which a heavy corrosive protection was applied were used for hull structures. The demand for speedy craft equipped with hydrofoils etc. has increased. Since high-speed sea water flow can come into contact with the hydrofoils, etc., such use prefers the use of a material which excels in sea water resistance without requiring being coated. In order to reduce hull weight further, a material having a high strength is preferred.
Although austenitic stainless steel can be important as a material which excels in sea water resistance, in a conventional production method, austenitic stainless steel is generally subjected to a solution annealing treatment after hot-rolling, thereby softening the resultant austenitic stainless steel so that the proof stress of the austenitic stainless steel is at most 400 MPa.
The strength can be increased by performing a hot-rolling processing under a specific temperature condition while omitting the solution annealing treatment, which has been described in Japanese Unexamined Patent Application, First Publication Nos. S. 60-208459, H. 2-97649, and H. 4-6214.
In particular, Japanese Unexamined Patent Application, First Publication No. H. 2-97649 describes a production method of an austenitic stainless steel having a high proof stress while maintaining a low-temperature toughness. However, the sea water resistance is not taken into consideration in this austenitic stainless steel while maintaining low-temperature toughness. Although Japanese Unexamined Patent Application, First Publication No. H. 4-6214 describes a production method of an austenitic stainless steel which has a high proof stress of not less than 500 MPa and excellent sea-water resistance, which includes performing a heat treatment on steel which contains 0.3% or more of N and 0.5 to 3.0% of Mo under a specific condition, there is no disclosure in this publication regarding the toughness. of the material
The official reports for Japanese Patent Publication Nos. 2783895 and 2783896 describe a production technique of an austenitic stainless steel with little softening of a weld part by adding a Nb-type element.
Cr, Mo, and N are elements which increase sea water resistance, and the corrosion resistance ranking in steel is determined by the formula: PI=Cr+3.3(Mo+0.5W)+16N as a pitting index. When the PI value of the component shown in examples of Japanese Unexamined Patent Application, First Publication No. H. 4-6214 is determined, it is approximately 32 in the minimum case, but as a stainless steel which gives a higher PI value (not less than 35), SUS836L and 890L (which contain 23% or more of Ni) are austenitic types, whereas SUS329J4L (which contains 5.5 to 7.5% of Ni) is a two-phase type.
Since two-phase-type SUS329J4L contains a ferrite phase, SUS329J4L has high proof stress. A two-phase stainless steel known as a super two-phase, in which Mo and W contents are increased has also been developed, and application thereof as a material with high hardness and high corrosion resistance has started. On the other hand, a high strength steel material of an austenitic-type high corrosion resistance stainless steel having a PI value over 35 has not yet been put in practical use.
Stainless steel is more susceptible to crevice corrosion when it is shaped into a crevice form than when it is not shaped i.e. flat. Therefore, in order to produce steel suitable for broad use in hull structures and which is low-maintenance, it is required to develop a highly corrosion-resistant steel material which is higher than the steel material described in Japanese Unexamined Patent Application, First Publication No. H. 4-6214.
On the other hand, the demand for a stainless steel material for ocean-going craft which is reliable when stranded or after a collision between shipping is increasing. Characteristics of both the base material and the weldability are preferred for reliability. Regarding the reliability of the base material, high toughness is preferred in preparation for a collision. Among Cr, Mo and N, which increase corrosion resistance, as for Mo and Cr, it may not be sufficient to simply add, because processability in hot-rolling will significantly decrease likely due to the influence of delta ferrite contained in cast steel or semi-finished products. In addition, in the case of a high Cr and Mo steel, in general, the toughness of the steel deteriorates remarkably due to the influence of an intermetallic compound known as a σ phase, and hence it is necessary to add a large amount of Ni in order to suppress the influence of both. However, considering the rising prices of raw materials of Ni and Mo these days, development of a low-cost, highly corrosion-resistant stainless steel is especially desired. It should be noted that, two-phase steel may not be adopted because of its low-temperature toughness.
On the other hand, as for adding N as described in Japanese Unexamined Patent Application, First Publication No. H. 4-6214, it may be effective for maintaining the strength, however, excessive N causes the generation of bubbles at a welded part, thereby it may decrease the bonding strength and reliability of the welded part, to the contrary.
Thus, it is one of the objects of the present invention to provide an austenitic stainless steel hot-rolled steel material which has sea-water resistance and strength superior to the conventional steel, while maintaining low-temperature toughness, which is required in a structural member of a high-speed ship. Another object of the present invention is to provide an austenitic stainless steel hot-rolled steel material which excels in the properties of corrosion resistance, proof stress, and low-temperature toughness.
The strength, the toughness, and the corrosion resistance of a hot-rolled plate obtained by casting, heat-rolling processing has been reviewed, and it has been determined that it may be preferable to provide a heat treatment of an austenitic component system in which the N amount is not more than 0.35% in view of weldability and the PI value is not less than 35, in view of weldability. In particular, it has been determined that the toughness cannot be determined by only the Ni content, but is determined by the content of intermetallic compounds, which are contained in a steel material, having high Cr and Mo contents. The formation of a metallographic structure as such starts from the solidification of steel, in addition, the formation may be generated at any steps in hot-rolling processing. In particular, the influence of a chemical composition on a solidified structure has been investigated, and the influence of conditions on rough rolling of cast steel, homogenizing heat treatment, hot working, and heat treatment has been reviewed. As a result, it was determined to restrict the content of component elements the solidification structure and the metallographic structure of a steel material to obtain an austenitic stainless steel which can address the problems of the conventional technique and excels in corrosion resistance, toughness, strength, and hot processability, the solidification structure, the metallographic structure of a steel material, thereby completing the austenitic stainless steel of the present invention and the production method thereof.